| Summary: | There is strong evidence that variations in seawater chemistry occurred during the Phanerozoic Eon. Of particular importance are the changes in the Mg2+/Ca2+ ratio because they have been attributed to the oscillations between “calcite” and “aragonite seas” over time. In addition to the Mg2+/Ca2+ ratio variations, there were also major changes in pCO2 levels and alkalinity that could also affect the calcium carbonate (CaCO3) polymorph that precipitates from seawater. Experiments were conducted in seawater where the initial alkalinity and pCO2 levels were varied and then slow degassing of CO2 resulted in a gradual increase of saturation state with respect to CaCO3 and eventually nucleation. The pH was continually monitored throughout the experiments and it was used in combination with the initial alkalinity to calculate the pCO2 and saturation state of aragonite (sigmarag) at the time of nucleation. The morphology and mineralogy of the precipitates were determined using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, respectively. It was observed that the initial alkalinity greatly affected the nucleation pCO2 value and the CaCO3 polymorph that was precipitated. In seawater with Mg2+/Ca2+=1.2 and ~10 mM alkalinity and a pCO2 below 2,500 μatm, calcite that was overgrown with aragonite was the dominate polymorph nucleated, while pure aragonite precipitated when the pCO2 was above 2,500 μatm. Seawater with Mg2+/Ca2+=1.2 and a wide range of initial alkalinities (5-50 mM) produced variable results. Seawater with Mg2+/Ca2+=1.7 produced only aragonite at lower alkalinities, but calcite was nucleated when the alkalinity and pCO2 values were exceptionally high, typically above 11 mM. These results point to the need to also consider the effects of the carbonic acid system in the “critical” Mg2+/Ca2+ region of about 1 to 2 for “calcite seas” and “aragonite seas” at various times throughout the Phanerozoic Eon.
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